More robust feature reordering

Two issues:

1. carrierwave always considers images invalid in prod because
the file has been moved by the virus scanner. This was preventing the
ordering from being updated
2. we were swallowing/silently ignoring errors on reordering which made
this hard to diagnose

I've addressed (1) by only requiring image on create on features.

I've addressed (2) by making error checking more rigorous on reorder and
propogating those errors.
Always show localized label for features in admin